1. Field of the Invention
The present invention relates to photoelectric conversion apparatuses, and particularly relates to a read circuit of a photoelectric conversion apparatus.
2. Description of the Related Art
In general, photoelectric conversion apparatuses are used in digital still cameras. For example, photoelectric conversion apparatuses employing CCD sensors or MOS photoelectric conversion apparatuses are typically used. Such a MOS photoelectric conversion apparatus includes a pixel unit including basic cells (pixels) having photoelectric conversion elements such as photodiodes, the basic cells being arranged in a plane, a holding unit that holds signals transmitted from the pixel unit, and a common signal line (horizontal signal line) used to output the signals supplied from the holding unit to an external device.
Improvements have been made to photoelectric conversion apparatuses to increase the number of pixels and to thereby meet a demand for a larger format. Accordingly, the number of switching transistors has increased (that is, total source capacitance has increased) and the capacitance of a signal line has increased, resulting in an increased parasitic capacitance of the common signal line. Japanese Patent Laid-Open No. 2003-224776 discloses a photoelectric conversion apparatus having a configuration in which a signal supplied from a pixel unit is read to block lines and thereafter output to a common signal line so that the capacitance of the common signal line is reduced.
Meanwhile, to address the increased number of pixels, there has been a demand to increase a speed of a signal reading operation. The speed of the signal reading operation is determined in accordance with the capacitance of the common signal line and a speed of a resetting operation of the common signal line. In the resetting operation of the common signal line, before or after a signal is read, a potential of the signal line is set to a predetermined potential. Note that Japanese Patent Laid-Open No. 10-191173 discloses such a technique.